[c] 1999, InterPress Third World News Agency (IPS)
June 8, 1999
By Gustavo Capdevila

GENEVA, Jun 8 (IPS) - Aircraft emissions are changing the concentration of gases in the upper atmosphere, contributing to climate change and increased global temperatures, says the World Meteorological Organisation (WMO).

Gases and particles given off by airplanes altered the concentration of greenhouse gases - such as carbon dioxide, ozone, and methane. The resultant condensation trails may increase the number of cirrus clouds; all of which contribute to climate change, the WMO explained.

A report released here, ''Aviation and the Global Atmosphere,'' was prepared by the Intergovernmental Panel on Climate Change (IPCC). The study was limited to providing scientific assessment, not policy recommendations, clarified IPCC secretary, N. Sundararaman.

The IPCC, an organisation created in 1988 by WMO and the United Nations Environment Programme (UNEP), prepared the report together with the Scientific Assessment Panel to the Montreal Protocol on Substances that Deplete the Ozone Layer at the request of the International Civil Aviation Organisation (ICAO).

The study underscored the rapid expansion of world aviation which, since 1960, has grown at an average annual rate of nine percent - 2.4 times greater than the world's average annual increase in gross national product (GNP).

An estimate used by IPCC researchers showed the world demand for airline passenger travel increased at a rate of five percent a year from 1990 to 2015.

The quantity of fuel used for air transport of passengers and cargo and for military operations was expected to show a three percent annual increase during the same period.

The projections assumed that improved technologies ultimately would reduce total emissions - calculated per passenger and per kilometre - and that ideal air traffic management would be achieved by the year 2050.

Without such advances, however, emissions would be much greater, the IPCC warned.

The principal aircraft outputs are greenhouse gases - carbon dioxide and water vapour - but nitric oxide and nitrogen dioxide (together known as NOX), as well as sulfur oxides and soot are also found in emissions.

Carbon dioxide, for example, remained in the atmosphere for approximately 100 years, meaning that aircraft emissions cannot be distinguished from emissions coming from other sources, the study said.

Other gases and particles did not last as long in the atmosphere and remained concentrated near flight routes, especially in the northern hemisphere's middle latitudes.

To determine the effects of emissions on the ozone layer, the study made the distinction between subsonic aircraft, that flew in the upper troposphere and the lower stratosphere (at altitudes of between nine and 13 km), and supersonic aircraft, which travelled through the stratosphere at altitudes of 17 to 20 km.

NOX emissions in the upper troposphere and lower stratosphere were expected to increase ozone and decrease methane. In contrast, at higher altitudes, the increased NOX is expected to deplete the stratospheric ozone layer.

The report stated that the prevalence of NOX gases, the precursors to ozone, increased with altitude. The ozone changes caused by aircraft depend on the altitude of the NOX released and varied according to regional changes in the troposphere and global changes in the stratosphere.

Researchers estimated that subsonic aircraft NOX emissions in 1992 resulted in a six percent increase in ozone concentrations at cruise altitudes in the northern mid-latitudes, compared to an atmosphere without aircraft emissions. The IPCC predicted the ozone increase would reach 13 percent by 2050.

These increases will tend to warm the Earth's surface, said the IPCC, an organisation created to evaluate information on various aspects of climate change.

Changes in the stratospheric ozone layer were caused, to a great extent, by the supersonic aircraft release of NOX, water vapour, and sulfur oxides. In addition, supersonic aircraft emissions in the northern hemisphere's stratosphere may travel to the southern hemisphere, creating further ozone depletion, stated the IPCC.

Sundararaman told a news conference that the effects of aviation emissions could be reduced through changes in aircraft and engine technology, as well as improved fuel and operational practices.

Subsonic aircraft currently being manufactured were 70 percent more efficient in fuel use per passenger and per kilometre than they were 40 years ago, according to the IPCC study.

It projected a 20 percent improvement in fuel efficiency by the year 2015, and scientists predicted a 40 to 50 percent improvement by 2050.

Increased engine efficiency would reduce fuel use and most types of emissions, but the condensation trails left by aircraft may increase, said the IPCC. Without advances in fuel technology, NOX emissions may also increase.

The report acknowledged that there would not be any practical alternative to the kerosene-based fuels used by commercial aircraft for the next few decades.

Other fuel options, such as hydrogen, may be viable in the long term but would require new aircraft designs and a new supply infrastructure. Hydrogen-based fuel would eliminate carbon dioxide emissions but would increase the water vapour released by aircraft, according to the IPCC.

The report concluded that the technological improvements in aircraft and engines, and in the air traffic system, would benefit the environment, but would not totally offset the effects of increased emissions resulting from aviation's predicted expansion.

Among options being discussed to further reduce emissions were stricter regulations on aircraft engine emissions and the elimination of subsidies and incentives that had negative environmental consequences.

Other policy alternatives mentioned by the IPCC included eliminating emissions trading and encouraging voluntary agreements, research programmes and substituting aviation with rail or bus travel. (END/IPS/tra-so/pc/mj/ld/mk/99)